1. Field of the Invention
The present invention relates to electronic semiconductor devices, and, more particularly, to quantum well devices in which carrier resonant tunneling through the well is modulated.
2. Description of the Related Art
Quantum well devices are known in various forms, heterostructure lasers being a good example. Quantum well heterostructure lasers rely on the discrete energy levels in the quantum wells to achieve high efficiency and typically consist of a few coupled quantum wells; see, generally, Sze, Physics of Semiconductor Devices, 729-730 (Wiley Interscience, 2d Ed 1981). High Electron Mobility Transistors (HEMTs) are another type of quantum well device and typically use only one half of a quantum well (a single heterojunction) but may include a stack of a few quantum wells. The HEMT properties arise from conduction parallel to the heterojunctions and in the quantum well conduction or valence subbands; the conduction carriers (electrons or holes) are isolated from their donors or acceptors and this isolation limits impurity scattering of the carriers. See, for example, T. Drummond et al, Electron Mobility in Single and Multiple Period Modulation-Doped (Al,Ga)As/GaAs Heterostructures, 53 J. Appl. Phys. 1023 (1982). Superlattices consist of many quantum wells so tightly coupled that the individual wells are not distinguishable, but rather the wells become analogous to atoms in a lattice. Consequently, superlattices behave more like new types of materials than as groups of coupled quantum wells; see, generally, L. Esaki et al, Superfine Structure of Semiconductors Grown by Molecular-Beam Epitaxy, CRC Critical Reviws in Solid State Sciences 195 (April 1976).
Resonant tunneling devices are the simplest quantum well devices that exhibit quantum confinement and coupling and were first investigated by L. Chang et al, 24 Appl. Phys. Lett. 593 (1974), who observed weak structure in the current-voltage characteristics of resonant tunneling diodes at low temperatures. More recently, Sollner et al, 43 Appl. Phys. Lett. 588 (1983), have observed large negative differential resistance in such devices (peak-to-valley ratios as large as six to one have been obtained), and Shewchuk et al, 46 Appl.Phys.Lett. 508 (1985) and M. Reed, to appear, have demonstrated room temperature resonant tunneling.
A typical resonant tunneling diode structure is schematically illustrated in FIGS. 1A-D; FIG. 1A is a schematic cross sectional view, FIG. 1B illustrates the profile of the conduction band edge through such a diode with no bias, FIG. 1C is the conduction band edge for bias into resonance, and FIG. 1D is a typical current-voltage characteristic for the diode at low temperature. The preferred material is the lattice matched system of GaAs/Al.sub.x Ga.sub.1-x As, although resonant tunneling has been observed in strained-layer heterostructure systems; see Gavrilovic et al, to appear. The two Al.sub.x Ga.sub.1-x As layers that define the central GaAs quantum well (see FIGS. 1B-C) serve as partially transparent barriers to electron transport through the diode. Resonant tunneling occurs when the bias across the outer terminals is such that one of the quantum well bound states has the same energy level as the input electrode Fermi level. Peaks in the electron transmission (current) as a function of bias (voltage) are thus observed. The resonant tunneling diode is the electrical analog of a Fabry-Perot resonantor. Leakage (inelastic tunneling current) is determined by the quality of the GaAs/Al.sub.x Ga.sub.1-x As interfaces and electron-phonon scattering.
The resonant tunneling diode has high speed charge transport (less than 100 femtoseconds) which implies applications to microwave oscillators and high speed switches. But the utility of such diodes is limited since they presently only exist as two terminal devices. The technology to contact the central quantum well (a third terminal) has not been demonstrated and, consequently, only two terminal diodes have been investigated. However, conventional semiconductor integrated circuit technology indicates that three-terminal devices will be necessary to build usable quantum well device systems. And fundamental problems arise with the obvious approaches to such three-terminal devices as described in the following.
A three-terminal resonant tunneling device requires a way to control the current through the device with a voltage or current supplied to the control terminal; the current through the device is presumed to be conducted by resonant tunneling of electrons. Now the obvious approach is to try to manipulate the potential of the quantum well. If this is to be done through the electrostatic potential, then mobile charges must be added to or removed from the device by the control terminal and these mobile charges will act as sources of the perturbation in the potential. However, the available mobile charges in semiconductors are electrons and holes, and both have problems. First consider what happens when holes are used to control the tunneling current. Because we want to modify the potential in the quantum well, the holes should be placed in the well formed by the valence-band discontinuity; see FIGS. 2A-B illustrating a three-terminal device (with the conduction current terminals labelled "emitter" and "collector" and the control terminal labelled "base" in analogy with bipolar transistors) and the conduction and valence band edges through such a device. To achieve resonant tunneling, the potential of the bottom of the quantum well must be biased below the conduction band edge in the emitter (source) terminal. Note that this implies that the emitter to base bias must be greater than the narrower band gap and in the forward direction for current flow. Now contact with the holes in the quantum well implies a bulk region that is doped p type. If this p region is in contact with the n type emitter, catastrophic current will result. Making the p type contact out of a higher bandgap semiconductor material (by selective epitaxy) does not solve the problem because the forward voltage of the parasitic p-n heterojunction is determined by the smaller bandgap.
The use of electrons to control the quantum well potential poses a different set of problems, which are closely related to those of the more conventional hot-electron transistors. See J. Shannon and A. Gill, High Current Gain in Monolithic Hot Electron Transistors,. 17 Electronics Letters 620 (1981); J. Shannon, Calculated Performance of Monolithic Hot-Electron Transistors, 128 IEEE Proceedings 134 (1981); M. Hollis et al, Importance of Electron Scattering with Coupled Plasmon-Optical Phonon Modes in GaAs Planar-Doped Barrier Transistors, 4 IEEE Electron Device Letters 440 (1983); and, generally, Sze, Physics of Semiconductor Devices 184 (Wiley-Interscience 2d Ed, 1981). To make such a device work we must essentially distinguish between the electrons controlling the current and those carrying the current, but electrons are indistinguishable particles, and thus the device must keep the two groups of electrons separated. As a particular case of this problem, consider the device shown in FIG. 3A, which includes a quantum well that is separately contacted to provide the control terminal, and its conduction band edge along lines A--A and B--B and shown in FIGS. B-C. Any electrons supplied to the quantum well from the base contact can readily tunnel out of the well. If we tried to raise or widen the barrier on the collector side to confine these control electrons, we would cut off the desired current of tunneling electrons. Note that the possible solution to this parasitic base-collector current would initially be to buffer the base region from the collector with a high bandgap region as shown in FIG. 3D. Here the same criticsim as before applies to current flow along the line C--C, though considerably less in this case due to the relative cross section of emitter-collector to base-collector. The balancing of constraints in such a device makes the possibility of successfully modulating the quantum well potential doubtful.
Also of interest is Esaki, Eu. Pat. Appl. No. 82100162.5, published Jan. 5, 1983, which tunnels electrons from an ohmic contact through an Al.sub.0.7 Ga.sub.0.3 As n.sup.++ doped emitter, an excited level in a GaAs n.sup.+ doped quantum well base, an Al.sub.0.7 Ga.sub.0.3 As n.sup.++ doped collector to an output ohmic contact. An ohmic contact is made to the base, but the problem of segregating the controlling and controlled carriers is not considered. Also, the base carriers can merely tunnel out into the collector and vitiate any base control.
Clearly, we need a better method of separation of controlling electrons from controlled electrons than that of the device of FIG. 3. Conventional semiconductor technology suggests confinement of the controlling electrons within a metal, preferably behind a high energy barrier (insulator). A possible such device, analogous to a field effect transistor, is illustrated in FIG. 4A with the conduction band edges along lines B--B and C--C shown in FIGS. 4B-C. The quantum well layer is accessed by tunneling source and drain contacts on the top side of the layer, and the gate establishes the bias on the quantum well, thus controlling tunneling into the quantum well. The device is biased so that electrons tunnel from the source into the well, drift to the regions under the drain, and tunnel out. The operation of the device raises questions of transverse transport in quantum wells; additionally, the large area gate will create excessive capacitance, which will limit the speed of operation.
In short, there is a problem to maintain the distinction between controlled and controlling carriers for three-terminal quantum well resonant tunneling devices.